Conventionally, the cathode ray tube (hereinafter “CRT”) has been widely used as a display device for displaying characters and images. However, due to the CRT's large size and large power consumption, the liquid crystal display device (hereinafter, “LCD”) has been widely spreading.
However, in displaying moving images on LCD, deterioration of image quality known as streaking or blurring is caused due to the display principle as described below.
In CRT, a dot-like electron beam is scanned to perform display. This is an impulse-type display in which light is emitted only momentarily at a given location of the display screen. On the other hand, in LCD, light always illuminates the liquid crystal panel with a backlight or the like. This is a hold-type display in which the display, except in black display, is continuous at a given location (pixel) of the display screen.
In the impulse-type display, there is no ghosting because the eyes track the instantaneous motions of moving images. On the other hand, in the hold-type display, since the display is continuous, it appears as though the same image were displayed at two different locations at one instance. This causes streaking and blurring. Further, the extent of streaking and blurring varies depending on the displacement of a displayed object between frames. That is, the extent of streaking and blurring varies depending on the speed of the observer's viewpoint tracking the displayed object. This is particularly prominent in large LCD.
As to the deterioration of moving image quality caused by failed gradations resulting from an insufficient response speed of the liquid crystal, the deficiency can be alleviated by increasing the response speed of the liquid crystal. However, since the display mode remains as the hold-type, the deterioration of moving image quality cannot be overcome completely simply by increasing the response speed of the liquid crystal.
One way to solve this problem is to provide an illumination period and a non-illumination period for the liquid crystal panel within one frame period, and thereby provides an impulse-type-like display mode from the hold-type display mode. Further, since gradation data is written by scanning each pixel of the liquid crystal panel line sequentially, the response of the liquid crystal completes at different times on the display screen. In order to prevent deterioration of image quality due to a response delay of the liquid crystal, there has been proposed a method in which a plurality of illumination regions are provided on the backlight and these illumination regions are sequentially illuminated to achieve impulse-type-like display.
For example, as illustrated in FIG. 19, Patent Document 1 discloses a display device using a direct-type backlight for a single-panel liquid crystal display section 300 (liquid crystal panel), wherein the backlight houses a plurality of light sources 301 in a housing that also serves as a light reflector 302. In the display device, the backlight is divided into a plurality of illumination regions with the light sources 301, so that each light source 301 can be turned on and off for each illumination region according to the response state of the liquid crystal display section 300. This is known to improve the quality of moving images by causing the liquid crystal display section 300 to display moving images in an impulse-type-like display mode.
Patent Document 2, another conventional example, discloses a display device using a side edge-type backlight, in which a polymer dispersed-type liquid crystal interposed between transparent electrodes is disposed on a back side of a light guiding plate. In the display device, the transparent electrodes are formed in stripe patterns, and, under applied voltage, the refractive index of the liquid crystal layer is changed so as to control passage and scattering of light propagating through the liquid crystal layer. With this control, the liquid crystal panel can be illuminated by scattering light in regions selected by the stripe transparent electrodes. This is known to improve the quality of moving images in an impulse-type-like display mode.
In a different technical area, a display device has been proposed in which a light guiding plate and actuators are used, as disclosed in Patent Document 3 for example. In the display device, a light source is provided on an end surface of the light guiding plate, and the actuators are provided on the opposite surface of the display surface of the light guiding plate. The actuators are used to emit light that propagates through the light guiding plate. In portions of the light guiding plate in contact with the actuators, the conditions of total reflection are not satisfied for the light propagating through the light guiding plate, and as such the light scattered by the actuators is emitted from the surface of the light guiding plate opposite the actuator. Further, in the display device, a multiplicity of micro actuators are provided to respectively correspond to the pixels, and gradations are realized according to the duration in which the actuators are in contact with or not in contact with the light guiding plate. In order to perform color display in the display device, it has been proposed to provide color filters for the respective actuators corresponding to the pixels.
[Patent Document 1]
Japanese Laid-Open Patent Publication No. 275604/2000 (Tokukai 2000-275604; published on Oct. 6, 2000)
[Patent Document 2]
Japanese Laid-Open Patent Publication No. 49037/2002 (Tokukai 2002-49037; published on Feb. 15, 2002)
[Patent Document 3]
Japanese Laid-Open Patent Publication No. 287176/1995 (Tokukaihei 7-287176; published on Oct. 31, 1995)
However, the foregoing conventional techniques have the following problems. With the plurality of illumination regions divided according to the plurality of light sources 301 as in the display device of Patent Document 1 for example, at least the same number of light sources needs to be provided for the number of illumination regions provided. This poses the problems of increased cost and increased device size. Further, since the illumination regions need to be divided by partitions or the like, the quantity of illumination light on the illumination plane becomes different between portions where the partitions are provided and portions where the partitions are not provided, with the result that luminance non-uniformity is caused.
Further, since the cold cathode tubes, the light sources 301, are turned on and off repeatedly, the light sources 301 have a short life. Further, this causes color breakup deterioration of image quality when the light source is turned off, owning to the fact that the fluorescence of different colors (e.g., red, green, blue) in the cold cathode tubes has different persistence times.
Further, in the display device in which polymer dispersed-type liquid crystal is disposed on a back side of the light guiding plate as in the display device of Patent Document 2 for example, the light propagates through the liquid crystal layer, which generally does not have good transmittance. As a result, a propagation distance of light is increased particularly in a large LCD, in which streaking and blurring are prominent. This leads to a large loss of light and poor light using efficiency.
Further, since the propagation distance of light before it leaves the light guiding plate is different between a portion near the light source and a portion far away from the light source, the quantity of light absorbed by the liquid crystal layer is also different. That is, the quantity of propagation light is different in different parts of the display plane, with the result that luminance non-uniformity is caused.
Further, by the time light reaches the liquid crystal inside the liquid crystal panel where images are displayed, the quantity of light has been reduced due to the passage through the polarizer. However, the quantity of light that propagates through the light guiding plate and the polymer dispersed-type liquid crystal and scattered therein is greater by more than 10 fold than the quantity of light that passes through the liquid crystal panel. Such a large light quantity causes degradation of the polymer dispersed-type liquid crystal. This is particularly made serious by the UV light contained in the light emitted from the cold cathode tubes.
Further, in the display device in which the light guiding plate and actuators are used as in the display device of Patent Document 3 for example, the actuators need to be provided to respectively correspond to the pixels. Further, the actuators need to be separated from one another to allow for independent control for each pixel. For these reasons, in the display device, there is a gap between the actuators. This reduces the numerical aperture and thereby the efficiency of using light. Further, the resolution is inferior compared with the LCD of the same screen size. Further, since gradations are realized only with the actuators, a fast operation is required for the actuators. This poses the problems of productivity and cost.